The present invention relates to a scanning lens whose lateral chromatic aberration is compensated by means of a combination of a refractive lens and a diffractive lens structure. The invention also relates to a scanning optical system that uses such a scanning lens.
These kinds of scanning optical systems are disclosed in, for example, U.S. Pat. No. 6,124,962. In the scanning optical system disclosed in the above patent, a beam emitted from a light source is deflected by a deflector (i.e., a polygon mirror), and is converged through a scanning lens (i.e., an fxcex8 lens) to form a spot on a surface to be scanned such as a surface of a photoconductive drum. The beam spot formed on the surface to be scanned moves (i.e., scans) on the surface in a main scanning direction as the polygon mirror rotates. The fxcex8 lens consists of three refractive lens elements. One lens surface of the refractive lens elements is formed with a diffractive lens structure to compensate a lateral chromatic aberration due to dispersion of the refractive lens elements. The diffractive lens structure is similar to a Fresnel lens. A large number of concentric relief patterns, each of which has a wedge sectional shape, are formed on a refractive lens surface. The relief patterns are symmetrical with respect to the optical axis of the fxcex8 lens.
In this specification, a direction equivalent to the scanning direction of the beam spot on the surface to be scanned is referred to as a main scanning direction, a direction perpendicular to the main scanning direction on the surface to be scanned is referred to as the auxiliary scanning direction. Shapes and orientations of powers of respective optical elements will be defined on the basis of these scanning directions. Further, a plane including the scanning beam scanning in the main scanning direction is referred to as a main scanning plane. The main scanning plane is perpendicular to the rotation axis of the deflector.
In the scanning optical system disclosed in the patent, the respective optical elements such as the light source, the polygon mirror and the fxcex8 lens are arranged such that a central axis of the beam incident on the polygon mirror exists in the main scanning plane and the beam incident on the polygon mirror travels along a path that is different from the optical axis of the fxcex8 lens.
However, the scanning optical system disclosed in the above US patent has such a weak point that the residual lateral chromatic aberration is asymmetrical and cannot be compensated. As disclosed in the patent, when the central axis of the beam incident on the polygon mirror exists in the main scanning plane and it travels along the path that is different from the optical axis of the fxcex8 lens, a deflecting point, which is the point of intersection of the central axis of the beam incident on the polygon mirror and a reflecting surface of the polygon mirror, moves in the direction of the optical axis of the fxcex8 lens and in the main scanning direction. The displacement of the deflecting point changes the lateral chromatic aberration on the surface to be scanned. Since the displacement of the deflecting point is asymmetrical with respect to the optical axis of the fxcex8 lens in the optical system disclosed in the patent, the residual lateral chromatic aberration becomes also asymmetrical.
It is therefore an object of the invention to provide an improved scanning lens that is capable of reducing the asymmetrical component of the lateral chromatic aberration by means of the combination of the refractive lens and the diffractive lens structure. A further object of the present invention is to provide an improved scanning optical system that employs the scanning lens being free from the asymmetrical lateral chromatic aberration.
For the above object, according to the invention, there is provided a scanning lens, including a refractive lens, which includes at least one lens element, having a positive power as a whole, and a diffractive lens structure that is formed on at least one lens surface of the refractive lens for compensating a lateral chromatic aberration caused by the refractive lens, wherein the diffractive lens structure is defined by an optical path difference function that is asymmetrical with respect to the optical axis of the refractive lens in the main scanning direction. The optical path difference function is represented by a polynomial having odd order terms.
When the above-described scanning lens is applied to a scanning optical system where a central axis of a beam incident on a deflector exists in the main scanning plane, the asymmetrical component of the lateral chromatic aberration due to the displacement of the deflecting point can be counterbalanced with the asymmetry of the diffractive lens structure.
Further, a scanning optical system, for a writing device such as a printer, of the invention includes a light source portion, a deflector that deflects a beam emitted from the light source portion, and the above-described scanning lens that converges the beam deflected by the deflector onto a surface to be scanned. It is preferable that a central axis of the beam incident on the deflector exists in the main scanning plane and it travels along a path that is different from the optical axis. In this case, while the absolute value of an additional optical path length determined by the optical path difference function increases with distance from the optical axis of the scanning lens in the main scanning direction, the gradient of the function in the side of the beam incident on the deflector is preferably greater than that in the other side with respect to the optical axis.
Still further, a scanning optical system, for a reading device such as a scanner, of the invention includes the above-described scanning lens through which a light from an object to be read passes, a deflector that deflects a light passed through the scanning lens, and a photodetector that receives the light deflected by the deflector at a fixed position. It is preferable a central axis of the light incident on the photodetector exists in the main scanning plane and it travels along a path that is different from the optical axis. In this case, while the absolute value of the additional optical path length determined by the optical path difference function increases with distance from the optical axis of the scanning lens in the main scanning direction, the gradient of the function in the side of the light incident on the photodetector is preferably greater than that in the other side with respect to the optical axis.